1. Field of the Invention
The present invention relates to driving force transmission devices that transmit a driving force by friction between a plurality of clutch plates.
2. Description of the Related Art
Conventionally, a driving force transmission device is known which is mounted on, e.g., a four-wheel drive vehicle having main driving wheels to which a driving force of a driving source is constantly transmitted and auxiliary driving wheels to which the driving force of the driving source is transmitted in accordance with the traveling state of the vehicle, and which can permit and cut off transmission of the driving force of the driving source to the auxiliary driving wheels (see, e.g., Japanese Patent Application Publication No. 2013-100079 (JP 2013-100079 A)).
The driving force transmission device described in JP 2013-100079 A includes: an inner shaft and a housing which can rotate relative to each other about the same axis; a case accommodating the inner shaft and the housing; outer clutch plates spline-fitted in the housing; inner clutch plates spline-fitted on the inner shaft; and cam mechanisms that generate cam thrust for the outer clutch plates and the inner clutch plates. Lubricant is present between the outer and inner clutch plates. The lubricant restrains wear that is caused by frictional sliding between the outer and inner clutch plates.
The case has a cylindrical accommodating chamber and a reservoir chamber. The housing and the outer and inner clutch plates are accommodated in the accommodating chamber. The reservoir chamber can temporarily store the lubricant contained in the accommodating chamber. The reservoir chamber opens to the accommodating chamber and is separated from the accommodating chamber by an annular partition wall that closes a part of the opening of the reservoir chamber. The reservoir chamber has a lubricant inlet port and a lubricant outlet port. The opening of the inlet port is not closed by the partition wall, and the outlet port is located downstream of the inlet port and discharges the stored lubricant into the accommodating chamber. The opening area of the outlet port is smaller than that of the inlet port.
When the driving force transmission device is not in operation and thus is not transmitting a driving force, drag torque may be generated due to viscosity of the lubricant that is present between the outer and inner clutch plates. In this driving force transmission device, a flow of the lubricant is formed in the rotational direction of the housing by a centrifugal force that is caused by rotation of the housing. The lubricant of this flow is introduced into the inlet port of the reservoir chamber and stored in the reservoir chamber. This reduces the amount of lubricant in the accommodating chamber accommodating the outer and inner clutch plates and thus reduces the drag torque that is generated when the driving force transmission device is not in operation.
In the driving force transmission device described in JP 2013-100079 A, however, the lubricant in the accommodating chamber is introduced into the reservoir chamber by the centrifugal force that is caused by rotation of the housing. Accordingly, for example, when the housing rotates at a low rotational speed, the amount of lubricant that is supplied to the reservoir chamber is reduced, and the lubricant remains in the accommodating chamber. The drag torque therefore may not be sufficiently reduced. The driving force transmission device described in JP 2013-100079 A therefore has room for improvement in terms of achieving further reduction in drag torque.